Do not overcook. Add pickles and continue cooking about 15 minutes.
In the meantime in a small bowl, beat milk and flour till smooth and stir in a small amount of the hot soup (to temper) and add to soup.

Bring to the boil and cook until slightly thickened.

Remove from heat.

In a small bowl, beat egg with sour cream till smooth and stir in a small amount of the hot soup.